200 tdi revs and stops

Hi, my 200TDi has major fuel problems. It will start but runs very jerky on tick-over. When you press the accelerator the engine momentarily revs but dies within a few seconds. I have changed the fuel filter and lift pump but it still will not go. the mother in law drove it for 25 miles like this, when running above 40 mph it runs ok but stops when revs drop, There were no other symptoms and the engine seemed to run fine before this happened - Has anyone got any ideas ????? Cheers